During spontaneous breathing, lightly anesthetized rats exhibit considerable breath to breath changes in respiratory pattern. This proposal addresses the hypothesis that much of this respiratory variability is due to an "instability" in mechanisms for the control of end-expiratory volume (EEV) in these animals with compliant chest walls. This "instability" is a form of complex irregular behavior in the nonlinear dynamics sense. We hypothesize that it is due to interactions of time-varying, nonlinear, pulmonary mechanoreceptor feedback with integrative central mechanisms for respiratory rhythm and pattern formation. In anesthetized, tracheotomized rats we have previously described how the sensitivity of respiratory phase transitions to vagal afferent activity causes breath to breath variability in the breathing pattern. This variability is accentuated during presumed lung deflation by continuous negative airway pressure (CNAP) and may be due partly to deflation-sensitive, slowly-adapting pulmonary receptors (DSARs). The present proposal will extend these studies by first relating respiratory variability directly to EEV. Next we will characterize DSARs, comparing them to pulmonary stretch and rapidly-adapting receptors. In addition both the above mechanisms which control EEV by modulating the dynamics of respiratory phase transitions and other mechanisms which modulate upper airway braking of expiratory airflow for EEV control are themselves modulated by pulmonary mechanoreceptor feedback; therefore, we also will study anesthetized rats with intact upper airways in order to investigate whether mechanisms which control EEV by modulating upper airway braking also induce respiratory pattern variability. Finally we will test the hypothesis that breath to breath variability in chemoreflex control of ventilation amplifies the respiratory pattern variability associated with EEV control. The proposed studies will extend current concepts about reflex control of respiration by characterizing the breath to breath variability of vagally-mediated reflex effects on respiratory pattern and elucidating some mechanisms which are expected to exaggerate this variability. Furthermore, analogies with respiratory behavior in human neonates suggest broader applicability of the expected findings.